He cheated on me with her, now she's feeling my pain.

I was in a terribly relationship for 4 years. I was 18 and now I'm 22, going on 23. The cheating was ridiculous and eventually, it became normal. At 18 I didn't know what I deserved and that's why I stuck around. We broke up 6 months ago and this June, I found the man of my dreams. He loves and adores me and he doesn't cheat. I hit the jackpot. Now, my ex has a new girlfriend and she recently messages me asking for help because he's cheating on her. I probably forgot to mention that he left me for her and she knew about me. My question is, should I help help her? And am I wrong for thinking she deserved it?